1. Field of the Invention
The present invention relates to a semiconductor device, and particularly, to a semiconductor module for processing superhigh-frequency signals that employ millimeter waves and microwaves as carriers.
2. Description of the Prior Art
Due to a shortage of frequencies to use for communications, superhigh-frequencies involving, for example, millimeter waves (30 GHz or over) are spotlighted.
FIG. 31 shows a module having an MMIC (monolithic microwave integrated circuit) chip for processing superhigh-frequency signals, according to a prior art. The MMIC chip 210 has, although not shown in the figure, active elements such as HEMTs (high electron mobility transistors) and concentrated or distributed constant circuits such as capacitors and resistors at the center of the principal surface of the MMIC chip 210. Around these elements, the MMIC chip 210 has bonding pads 230.
The MMIC chip 210 is mounted on a board 220, which has bonding pads 260. The bonding pads 230 and 260 are electrically connected to each other through bonding wires 240. Some of the bonding pads 260 are electrically connected to through holes 270 made through the board 220, and the through holes 270 are electrically connected to a ground electrode 280, which is substantially entirely formed over the back of the board 220.
A signal line 250 on one side of the board 220 guides external signals to a bonding wire 240 and a bonding pad 230 and then to the elements on the MMIC chip 210. The MIC chip 210 is square or rectangular in plan view. If the MMIC chip 210 is rectangular, the signal line 250 is usually connected to the center of a short side.
FIG. 32 shows examples of the elements formed in a central area of the principal surface of the MMIC chip 210. The elements may be HEMTs 201, capacitors 203, and distributed constant circuits 202a to 202d.
This prior art employs wire bonding to mount MMIC chip 210 on the board 220, and therefore, the chip and board must have the bonding pads 230 and 260. The bonding pads must be sufficiently large for connecting the bonding wires 240. In addition, the bonding pads on the chip and board must be separated from each other by a given distance to secure workability.
On the other hand, semiconductor modules for processing low-frequency signals frequently employ flip-chip mounting instead of the wire bonding. For the flip-chip mounting, metal bumps are formed in place of bonding pads on a chip, the chip is turned over, set on a board, and connected the bumps to signals lines on the board, to thereby electrically connect elements on the chip to the signal lines.
Compared with the wire bonding, the flip-chip mounting is advantageous in reducing a package area or volume. Since the flip-chip mounting uses no bonding wires, it causes no deterioration in signal transmission characteristics due to the inductor components of the bonding wires.
These advantages raise a need for applying the flip-chip mounting for superhigh-frequency MMIC modules.
It is not clear, however, if conventional flip-chip mounting structures for the low-frequency modules are applicable as they are to the superhigh-frequency modules. It is necessary, therefore, to provide novel design principles appropriate for the superhigh-frequency modules.